Count instances of String

I have a rollup field that uses ARRAYJOIN() to produce string values separated by commas. Next, I want to be able to count all instances of a particular string.For Example, the field I want to count contains: BT,DT,BT, DT. I want to count only the “DT.” Any suggestions on what formulas are able to do this? I would use a conditional rollup, but it doesn’t feature any “IF” statements.